Fluorescent screen



Nov. 24, 1953 c. w. PUTNAM 2,660,686

FLUORESCENT SCREEN Filed June 19, 1948 I I 5/? I 1 .5

WITNESSES: INVENTOR 4%! Charles W Putnam iatcnted Nev. 24,1953

illilTED STATES PATENT OFFlCE FLUDRESCEN'E SCREEN Application June 19,1948, Serial No. 34,066

13 Claims. l

My invention relates to radiation-emitting screens and, in particular,it relates to a process for making light-emitting screens which producea visible replica of the intensity distribution in an X-ray field. Oneuse for my invention is in electr cal discharge tubes in which electronimages corresponding to a light image generated on a fluorescent screenby X-rays are first produced, and then a second light image which is anintensified replica of the initial light image is produced by incidenceof the electron ima e on a second fluorescent screen. Certain subjectmatter herein disclosed is being claimed in my application Serial No.146,373, filed February 25, 1950, for a Fluorescent Screen.

One example of an electrical discharge device this type is shown andclaimed in Lloyd P. Hunter and Richard Longini ?atent 2,555,5 l5 for anImage intensifier, issued June 5, 1951. In the device just mentioned, anelectron image is produced which is a replica of an X-ray picture on afluorescent screen; the electrons are then accelerated to high velocityand into incidence upon a screen comprising a cathode ray phosphor, andthe light image produced on the latter is observed. In the electricaldischarge tube just described, the photo-electric surface comprisesmaterials such as cesium which. are active chemically and hav a vapor ofa substantial pressure and which likely to react chemically with thecomponents in the fluorescent screen during processing. It is,accordingly, necessary to provide some protective coating for thefluorescent material which is capable of preventing this reaction.

One object of my invention is, accordingly, to provide a novel type offluorescent screen in which the fluorescent material is provided with acoating which is substantially transparent to incident light and iscapable of preventing chemical reaction between the phosphor of thescreen and cesium vapor.

Another object of my invention is to provide a protective coating forthe particles comprised in a fluorescent screen which shall preventchemical reaction between them and vapors present in the space aroundthem.

Still another object of my invention is to provide an improved form offluorescent screen in which chemical reaction between the fluorescentmaterials and an adjacent layer of chemically active material isprevented.

A still more specific object of my invention is to provide a method ofprotecting zinc sulphide from chemical reaction with photo-electricallyactive materials.

A still further object of my invention is to provide a method ofpreventin chemical reaction between fluorescent zinc sulphide and thcomponents of cesiated antimony located in a common container therewith.

Still another object 01" my invention is to provide a fluorescent screenwhich is much less difficult to outgas than are prior art screens inwhich the individual fine particles of fluorescent material are exposedto the atmosphere of an evacuated container, and which contains aminimum of adsorbed vapors likely to gradually exude into suchatmosphere during operative life of the tube.

Other objects of my invention will become apparent upon reading thefollowing description, taken in connection with the drawing, in which:

Figure 1 shows a view in longitudinal section of an electrical dischargetube containing a fluorescent screen protected in accordance with theprinciples of my invention;

Fig. 2 is an enlarged view in section of the screen forming a wallportion of 1; and

Fig. 3 is a longitudinal section of a container in which the plate 5 isplaced at an intermediate point in the fabrication.

Referring in detail to the drawing, the fluorecent screen, in accordancewith my invention, comprises a layer 5A made up of particles of afluorescent material or phosphor, such, for example, as zinc sulphide orsilver-activated zinc sulphide, which is embedded in glass plate 5positioned within a container 4 of glass or other suitable material. Thecontainer i may be a vacuum-tight enclosure, such as the tube l in theabove-mentioned Hunter and Longini application. As is illustrated insaid application, the enclosure t may contain during the process of itsmanufacture a substantial pressure of the vapor or some photo-electricmaterial, such, for example, as cesium used to produce a layer 8 ofcesiated antimony deposited in close contiguity to the layer 5A. Thereis also located on another portion of the wall 6 of the container 4, alayer 9 of an electron phosphor such as zinc sulphide, which may beprovided with chemical protection such as is described and claimed inthe Patent 2,586,304 of J. W. Coltman and R. L. Longini for Protectionof Electron Phosphors, issued February 19, 1952. As has been statedabove, there is a likelihood of chemical reaction between thefluorescent material of the layer 5A and photo-electric material justmentioned, and

to prevent such undesired chemical reaction, the fluorescent layer 5A isembedded in the glass of the plate 5 by a procedure which will now bedescribed.

Referring to the drawing, the final desired product is a glass plate 5with a layer 5A of fluorescent material or phosphor sunk into the uppersurface until all the phosphor particles are completely enclosed in theglass. The method of achieving this result is as follows:

A glass must be used for the plate 5 which will not be detrimental tothe phosphor. The glass must not dissolve the phosphor, and it must notcontain substances that will react with or unfavorably activate thephosphor. For example, the phosphor to be used maybe a silver-activatedzinc sulphide which fluoresces blue under excitation. This material asreadily purchasa ble upon the market is notably subject to adverseactivation by very minute quantities of impurities.

The glass for plate 5- is up from chemicals containing impurities lessthan 0.001%. The glass contains the following components: K20, ZnO,B203. To give one instance, I have found a mix of 31 percent by weightof potassium oxide (K20) with 69 percent by weight of boric oxide (B203)to be a satisfactory one. However, I found mixes within the range of Oto 40 percent K20, to 60 percent ZnO and 100 to 40 percent B203 to besatisfactory for many purposes. The constituents were fused in aplatinum crucible at 1000 C. and poured into an aluminum mold. Besidestheir purity, these glasses are relatively low-melting, and thecompositions mentioned above had a considerable range of thermalexpansion coefficients.

The glass melt is evacuated in the molten state to remove all occludedgases, formed into a thin plate, annealed, and cooled. The layer A offluorescent material is settled onto one surface of this plate from asolution containing a suitable binder.

Although a number of binders, such as borates, boro-phosphates orsilicates of the alkali metals could probably be used, the one developedfor this process has certain desirable features. The solution used wasapproximately 5% by weight potassium borate in water. The potassiumborate was roughly of the formula: K2O.3B2O3 or KB305 and very pure.tion is rapidly dried, it first forms an elastic him. which. on furtherdrying becomes hard. Upon heating to fusion, this material forms a glasswhich will not devitrify under any heat treatment anticipated in thisprocess, and it is a glass which is readily miscible with the glass usedfor the backing of the phosphor.

A homogeneous suspension of phosphor in the 5 percent potassium boratesolution is placed in a container with the glass plate in the bottom.The amount of the phosphor mixture poured into the container will, ofcourse, determine'the thickness of the resulting layer. A layerthickness when dried of 12 mils will be found suitable for manypurposes.

The glass plate with superimposed phosphor layer is then baked atapproximately 125 C. to dry the binder.

The glass plate is then placed, phosphor layer up, on a support of sucha substance as graphite which, when the glass is heated to a softenedcondition, will maintain the glass shape, and Will not stick to it. Thissupport may have an integral heater, or it should be thin enough and ofa material which will readily conduct heat.

When a thin layer of this solu- The glass and support are then placed ina container which can be evacuated. The chamber is closed and evacuated.The temperature is gradually raised while under vacuum to a point justbelow the softening point of the glass. The purpose of this treatment isto complete the removal of water from the binder and to outgas thephosphor and the surface of the glass. The vacuum is then broken,allowing the chamber to return to atmospheric pressure. The potassiumborate binder protects the phosphor against oxidation. Zinc sulphideordinarily oxidizes at 600 C. in air.

The temperature is then raised above the softening point of the glass(approximately 900 C'.). This temperature is maintained while the moltenglass diffuses up through the phosphor layer until the upper surface ofthe bonded unit is a continuous, though slightly irregular, glasssurface. At this point the glass should be quickly reduced to annealingtemperature, annealed and cooled.

The cesiated antimony layer may be applied to the surface of thefinished plate by methods known in the art. Patent 2,456,968 of R. L.Longini for a Process for Outgassing Photocells Containing Antimony,issued December 21, 1948, shows one such method.

I claim as my invention:

1. A vacuum-tight container containing a glass support having afluorescent material completely submerged beneath its surface and a coating of photo-electric material on said support.

2. A vacuum-tight container containing a glass support having a layer.of zinc sulphide completely submerged beneath its surface and coatedwith photo-electric material.

3. A vacuum-tight container enclosing an atmosphere containing asubstantial pressure of cesium during manufacture and having a screencomprising a glass support having a layer of fluorescent materialcompletely submerged beneath its surface.

4. A vacuumtight container enclosing an atmosphere containing asubstantial pressure of cesium during manufacture and having a screencomprising a glass support having a layer of zinc sulphide completelysubmerged beneath its surface.

5. A vacuum-tight container enclosing an atmosphere containing asubstantial pressure of cesium during manufacture and having a screencomprising a support consisting of 0% to 40% potassium oxide, 0% to 60%zinc oxide, and to 40% boric oxide, said support having a layer of zincsulphide submerged within its surface.

6. A vacuum-tight container having a screen comprising a supportconsisting of 0% to 40% potassium oxide, 0% to 60% zinc .oxide, and 100%to 40% boric oxide, said support having a layer of zinc suiphidesubmerged within its surface.

'7. A vacuum-tight container enclosing an atmosphere containing asubstantial pressure of cesium and having a screen comprising a supportconsisting of 0% to 4.0% potassium oxide, 0% to 60% zinc oxide, and 100%to 40% boric oxide, said support having a layer of zinc sulphidesubmerged within its surface.

8. A vacuum-tight container enclosing an atmosphere containing asubstantial pressure of cesium and having a screen comprising a glasssupport having a layer of fluorescent material completely submergedbeneath its surface.

9. A vacuum-tight container enclosing an atmosphere containing asubstantial pressure of 5 cesium and having a screen comprising a glasssupport having a layer of zinc sulphide completely submerged beneath itssurface.

10. A vacuum-tight container containing a glass support having a layerof zinc sulphide complete- 1y submerged beneath its surface and coatedwith cesium.

11. An image screen comprising a support consisting of to 40% potassiumoxide, 0% to 60% zinc oxide, and 100% to 40% boric oxide, said supporthaving a layer of zinc sulphide submerged within its surface.

12. An image screen comprising a support consisting of 0% to 40%potassium oxide, 0% to 60% zinc oxide, and 100% to 40% boric oxide, saidsupport having a layer of phosphor submerged beneath its surface.

13. A vacuum-tight container enclosing an atmosphere containing asubstantial pressure of a component which is chemically active towardphosphors and having a screen comprising a support consisting of 0% to40% potassium oxide, 0% to 60% zinc oxide, and 100% to 40% boric oxide,said support having a layer of at least one of said phosphors submergedbeneath its surface.

CHARLES W. PUTNAM.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date Berger June Leverenz Mar. 2, Hartmann Aug. 3, Winninghoff Oct.5, Germer July 19, Schleede Nov. 15, Fairbrother Apr. 4, Massa et all.Oct. 31, Bruche et al. Nov. 7, Langmuir Apr. 23, Steadman Aug. Bode Apr.8, Scott et al. Apr. 7, Roberts Oct. 13, Hood et a1. Mar. 30, Hooley May14, Szegho July 8,

